Basic research on protein structure, function, regulation, and processes of folding/unfolding has produced the following results: (1) Urea-induced dissociation and complete unfolding of dodecameric Mn.glutamine synthetase (Mn.GS) from E. coli at pH 7 have been studied by spectral and calorimetric techniques. An enthalpy change of 14 +/- 4 cal/g with a proton uptake of approximately 50 protons/mol has been determined. The calorimetric data are consistent with urea binding to multiple, independent sites in Mn.GS and the number of binding sites increasing approximately 9-fold during protein unfolding. (2) Thermal unfolding of the holo-tryptophan synthase alpha-beta-beta- alpha complex (with bound pyridoxal phosphate (PLP)) from S. typhimurium has been studied. DSC and spectral data are consistent with a model of 6 sequential two-state transitions for the holo-alpha-beta-beta-alpha complex: 3 transitions at <65 degrees C involve exposure of Trp177 and a loss of ellipticity of bound PLP in beta chains followed by alpha chain unfolding and 3 transitions at >70 degrees C occur as Tyr residues are exposed and beta chains unfold (with a total heat of 750 +/- 25 kcal/mol). The cofactor increases both the linkage between unfolding domains and the stability of beta-beta subunits. (3) Thermally induced unfolding reactions in Acanthamoeba myosin II and intact rabbit skeletal muscle myosin have been characterized thermodynamically. Differences in the cooperative unfolding reactions of these myosins relate to differences in rod structures and possibly also head-rod interactions. ATP binding stabilizes head regions of both myosins. (4) Influenza virus hemagglutinin appears to adopt a molten globule conformation under fusogenic conditions (i.e., pH 4.9, 37 degrees C). From pH 7 to 5, a destabilization of tertiary structures and a stabilization of secondary structures are evident. (5) Ultracentrifugation has been used to determine ligand promoted shape and size changes of macromolecules and also for studies of interacting biopolymers. Hydrodynamic properties of the E. coli ATP- dependent Clp-AP protease in the absence and presence of ATP gamma S have been determined. Ligand effects on Acanthamoeba myosin II, glutamine synthetase, and actophorin-actin interactions also have been studied.